Focal Point is a GitHub project and CSS framework created by Adam Bradley. Focal Point not only resizes but crops images too.
More Compression (cowbell)
A case study showing that extra compression may hold the key to HD images
The Responsive Designer
A superb article by Simon Foster on his experiences in RWD
Hiding images for Responsive Design
This is probably the most efficient way of hiding an image for responsive design. Tim Kadlec recently did some excellent work testing a variety of browsers and their capacity to download assets specified by media queries.
When you need your type to respond like your site does, this jQuery plugin makes it happen.